An increase in immunoglobulin free light chains (FLC) was recently described in several pathological conditions, including asthma. FLC pathology is classically associated with monoclonal gammopathies. Its association with allergic disorders is surprising and unexplained. We therefore tested a cohort of children with severe atopic dermatitis (SCORAD 50-80) to determine the serum levels of free kappa and lambda chains, and correlated the results with clinical status and relevant laboratory markers. Seventy-three patients with severe forms of AD, all children from 3 months to 3 years of age and ninety healthy age-matched controls were included in the study. Light chains in sera were tested using the Freelite assay (Binding Site, Birmingham, UK). There were highly significant differences in both kappa (mean: 7.05 and 3.22 mg/l) and lambda (mean: 10.99 and 9.8 mg/l) serum levels between patients and controls, respectively (P < 0.0001). The kappa/lambda ratio in patients with allergy (mean: 0.64) was significantly higher than in controls (0.33) (P < 0.0001). We further observed significantly increased levels of FLC and their ratio in the group of patients with severe forms of AD in comparison to the group of patients with a resting stage of the disease or healthy controls (P < 0.05 and P < 0.0001, respectively). On the other hand, we could not confirm any association of FLC levels with age or total IgE levels. In conclusion, an increase in FLC reflects disease activity in children with severe atopic dermatitis. FLC might thus represent an additional diagnostic marker independent of total IgE levels.